In recent years, overlay networks have been spreading rapidly. This is because overlay networks enable the enhancement of security, improvement of service quality, and improvement of reliability.
Also, research on techniques for maintaining an optimal path as a path of the overlay network is ongoing (refer to Patent Document 1, for example). In the following, as an overlay network, a gateway device terminates the connection and determines the transfer path in response to the message header and body (i.e. data), assuming the overlay network of an application layer.
To maintain an optimal path of the overlay network, the data transfer control device that performs routing needs to periodically collect the link state between the gateway devices configuring the overlay network from each gateway device. Here, a link state is defined as the delay and the residual bandwidth of the TCP connection between the gateway devices.
When the data transfer control device directly collects the link state from all of the gateway devices configuring the overlay network, the load of the data transfer control device becomes very high. In other words, there is a problem in that the number of connections is very large. For example, when the number of the gateway devices is a thousand units, a thousand connections will be established with the data transfer control unit regularly. Generally, when the number of connections becomes on a scale of several thousand to tens of thousands, the performance of the devices is significantly deteriorated, which is problematic in terms of scale.
To reduce the number of connections, when the data transfer control device collects the link state, it is conceivable to transfer the link state via the gateway device along the path on the above-mentioned overlay network. According to this method, for the data transfer control device, even when the link state from either gateway is received, it may be enough to establish at least one connection with the gateway devices serving as the last hop on the path, thereby making it possible to solve the problem of the number of connections.
However, in the course of transferring the link state via the gateway devices along the path on the overlay network, when an abnormality such as a delay and a GW failure occurs, the link state does not reach the data transfer control device. Therefore, the data transfer control device cannot detect the abnormality between the gateway devices, and thus there is a problem in that an optimal path cannot be continuously maintained.
Patent Document 1: Japanese Patent Application Laid-Open No. 2009-10438